1. Field of the Invention
The present invention relates to the development of relatively inexpensive materials and means to apply them which are eminently suitable for the correction of iron deficiency-induced chlorosis in plants. More particularly, the present invention relates to the development of certain combinations of gel-forming hydrophilic polymers and of iron-containing fertilizer or fertilizer amendment materials (FAM) having characteristics which significantly increase their efficiency to correct iron deficiency-induced chlorosis in plants. Still more particularly, the instant invention relates to the discovery of certain aqueous gels adapted to encapsulate, isolate, or otherwise contain selected iron compounds to provide an economical and readily available iron source imminently suitable for correcting iron deficiencies in plant life growing at such situs. Even still more particularly, the instant invention relates to the discovery that such combinations of polymers and iron-containing fertilizer or iron source FAMs should be band applied at or prior to planting or spot placed in the root zone of growing plants in soil to minimize the contact of these products with the soil so that chemical reactions which adversely affect the availability of these products to plants are minimized.
2. Description of the Prior Art
A. First Embodiment--Consideration of Chlorosis. Iron is an essential element in plant nutrition and generally is classified as a micronutrient. It is known to be involved in the synthesis of chlorophyll which in turn is required for photosynthesis in plants. A deficiency of this micronutrient in growing plants, which can be greatly exaggerated in calcareous type soils, is oftentimes the cause of chlorosis, which is characterized by a yellowing of plant leaves and stems and which results in particularly poor growth.
Currently available practices for alleviating such iron deficiencies in growing plants include the application of synthetic iron chelates to soil or the use of various soluble iron compounds as foliar sprays for direct application to the plants. Currently, the least expensive, in terms of up-front per unit cost, water-soluble iron compound in use is iron sulfate, either in its reduced state, e.g., (FeSO.sub.4) or in the ferric state, e.g. [Fe.sub.2 (SO.sub.4).sub.3 ]. However, FeSO.sub.4 or Fe.sub.2 (SO.sub.4).sub.3 should not be applied directly to soil lest either source quickly becomes combined with certain components in the soil to form water-insoluble compounds thereby rendering such iron unavailable to growing plants.
The synthetic chelate FeEDDHA [ferric chelate of ethylenediamine (di-(o-hydroxyphenyl acetate))] has been deemed to be the most effective iron fertilizer for soil application, especially in calcareous soils (Arthur Wallace, A Decade of Synthetic Chelating Agents in Inorganic Plant Nutrition, Edwards Brothers, Inc., Ann Arbor, Mich. 1962). However, the per unit cost or iron in FeEDDHA is quite high, which makes this iron chelate material much too expensive for application to relatively low-value field crops. Another currently available and somewhat less expensive iron chelate material, FeEDTA (monosodium ferric ethylenediamine tetraacetate), has proven to be effective for crops growing in near neutral soils but not in calcareous, high-pH soils wherein most iron deficiencies occur. Nevertheless, the initial per unit cost of iron in iron sulfate is significantly lower than in FeEDDHA. Accordingly, iron sulfate would be more economical FAM eminently suitable for field crops if it remained available to growing plants subsequent to its contact or juxtapositioning with the soil situs. Therefore, additives or conditions which can significantly improve the effectiveness of iron sulfate for the treatment of chlorosis could, in turn result in an economically effective iron source FAM for soil application.
Currently, it is the practice in the trade for iron-containing or iron source FAMs to either be applied to soil separately or to be incorporated with other materials in the processing or blending of fertilizers. The effectiveness of iron source FAMs in maintaining a supply of iron to growing plants depends upon the chemical nature of the iron source materials and/or the soil, as well as their rate and/or frequency of application. Economic considerations regarding the use of iron source FAMs are determined by their costs and rate of application relative to the returns attributable to increased yields of the crops to which they are applied. Presently, the most effective iron chelate, FeEDDHA, is so costly that its use is restricted to high-cash value crops such as, for example, apples, grapes, and peaches, while the least costly, on a front-end per unit cost basis, iron source FAMs are ineffective when used in procedures designed to correct iron chlorosis in many lower value crops, such as, for example, grain sorghum and soybean.
From the aforesaid, it should now be abundantly clear that the prior art materials designed as, or intended to be, iron source FAMs are either too costly up front to be economical for use on most field crops or although available at relatively low unit cost, are still highly uneconomical to use since they are ineffective in maintaining a supply of available iron to crops growing on iron-deficient soils.
For the past 30 years, gel-forming hydrophilic polymers have been used to improve aggregation of soil particles as well as improving the storage capability and efficiency by which plants use water. One unique physical property of hydrophilic polymers is that they will retain up to 500 times their own weight of water in their chemical structure (Michael Johnson, The effect of gel-forming polyacrylamides on moisture storage in sandy soils, Journal of Science of Food and Agriculture 35:789-793, 1984). While this water can be removed rather rapidly from these polymers by applying heat thereto or rather slowly therefrom by allowing same to evaporate therefrom in the open air, these polymers will normally retain this water for long period of time in moist soil. Early use of such polymers in agriculture declined because these products, mainly starch based co-polymers and early prototypes of crosslinked acrylamide co-polymers were quite susceptible to microbial decomposition and had poor salt buffering properties (Michael Johnson and Cornelius Veltkamp, Structure and functioning of water-storing agricultural polyacrylamides, Journal of Science of Food and Agriculture 36:789-793, 1985).
Recent research and development of controlled release nitrogen fertilizers has included use of hydrophilic polymers as carriers of nitrogen solutions. Results of leaching column studies with soil-applied urea-ammonium nitrate showed that nitrate leaching losses were delayed up to 6 weeks when various polymer gels were included (Robert Mikkelsen and David Behel, Jr., Gelled fertilizers as slow/delayed release nutrient sources, Agronomy Abstracts, American Society of Agronomy Annual Meeting, Anaheim, Calif., p. 303, 1988). In a more recent study, gels of three polymers (polyacrylate, vinyl-alcohol, and starch-based) containing various nitrogen fertilizers released most of the contained nitrogen after only 7 days, but some of the nitrogen was retained by the gels up to 28 days; the water-holding capacity of each polymer varied with nitrogen source and concentration of the fertilizer solution (Jonathan Smith and Helen Harrison, Evaluation of polymers for controlled release properties when incorporated with nitrogen fertilizer solutions, Communications in Soil Science and Plant Analysis 22:559-573, 1991).
Results of investigations leading to the making of the instant invention have led to the unexpected discovery that the use of hydrated polymers as a matrix for soluble iron fertilizers provides a novel method for improving the efficiency of iron uptake by plants in calcareous soils. In the practice of a principal embodiment of the instant invention aqueous solutions of FeSO.sub.4, Fe.sub.2 (SO.sub.4).sub.3 or other soluble inorganic iron salts are absorbed by polymers to form gels which are then applied to soil, most preferably by banding. Because some polymers herein tested, are only slowly biodegraded in soil, it is anticipated that hydrogels of these materials will provide available iron for crops over a long period of time. A parameter which appears to have principal effects on the practice of the instant invention is the realization that the water-holding capacity of polymers may be decreased with increased Fe concentrations in the solution being absorbed because soluble salts decrease water absorption of gel-forming polymers (Michael Johnson, Effect of soluble salts on water absorption of gel-forming soil conditions, Journal of Science of Food and Agriculture 35:1063-1066, 1984). This could limit the concentration of iron in the solution, or require higher polymer concentrations to provide an adequate gel.
B. Second Embodiment--Micronutrient Delivery System It has been recognized that there is an increasing need for supplying micronutrients to crop plant-soil situses, because the natural supply of such micronutrients is insufficient to produce high crop yields in many of the soils in this country. This need has been accentuated by use of high-analysis fertilizers containing relatively small amounts, if any, of many micronutrient sources being removed from the soil. In addition to the need for micronutrient iron, as discussed in detail, supra, there are principal needs for amounts of many other micronutrients, some of the principal ones being zinc, copper, and manganese. The need for considering these and other micronutrients is documented in Micronutrients in Agriculture, 2nd, Ed., John Mortvedt, et al. (eds) Soil Science Society of America, Madison, Wis., 1992.
One approach to supplying such micronutrients to the proper soil situs has been to incorporate same in liquid or solid fertilizers during the production thereof, to thereby utilize the resulting micronutrient-enriched fertilizers as carriers or delivery systems of same to the crops. Early work by Stinson, et al., as reported in U.S. Pat. No. 3,244,500, Apr. 5, 1966, assigned to the assignee of the present invention, recognized that if water-insoluble micronutrient sources are utilized for incorporation in liquid fertilizers they unfortunately remain in water-insoluble form and, therefore, unavailable to the plants in which they are thereby delivered. Accordingly, Stinson, et al., teach that the water-soluble sulfate salts of micronutrients, which apparently became insoluble reaction products when they were incorporated in solid fertilizers at the time of their invention, would remain in water-soluble form in liquid fertilizers if the phosphoric acid utilized for the production thereof contained prescribed amounts of so-called polyphosphoric acids.
In that area of the art devoted to incorporating of micronutrients with solid fertilizers, such as in granular form, there evolved three general methods by which to add micronutrients to granular macronutrients, namely, dry blending, incorporation during granulation, and coating onto the surface of the finished fertilizer granules. An example of an improvement in the last approach is shown in U.S. Pat. No. 3,423,199, Philen, et al., Jan. 21, 1969, assigned to the assignee of the present invention, where in effect Philen, et al., teach the coating of hygroscropic fertilizer granules with micronutrient powders, which when wetted with water and/or steam react with the fertilizer constituent at the granule surface to form in situ stabilized compounds. The resulting complex fertilizer granules are thereby improved in their physical property characteristics and also act as delivery systems for micronutrient fertilizer elements.
Such heretofore devised micronutrient delivery systems have been fairly successful in that they place the materials to be delivered in a form suitable for use with either commercially available liquid fertilizer application equipment or commercially available solid granular application equipment. In addition, the incorporation of micronutrients in or with the fertilizers makes the delivery thereof to the farmer more convenient since he is not required to make a separate trip to the fields for application of fertilizer and also for application of micronutrients. However, it will be appreciated that these types of delivery systems utilize the so-called shotgun approach which require the use of substantial amounts if micronutrients and result in distribution of same to nondeficient areas of the field, it being understood, of course, that if the solid or liquid fertilizers are band applied, this somewhat wasteful practice is more limited. It will be further appreciated that there are instances wherein it will be highly desirable to be able to deliver and place the micronutrient at a situs deemed optimum for a particular plant or crop. Since the second or alternate embodiment of the instant invention is directed to the placement of micronutrients within islands or veins of hydrated polymers enriched with same, and further since it has now been discovered that once the plant roots find such a region there is a propensity for exaggerated root growth and development into and throughout said region, it can now be appreciated that this new delivery in effect focuses or concentrates or directs a substantial portion of a plants root development in a manner to most effectively and efficiently utilize such micronutrients.